StreamDecoder: fix support for out-of-order legacy store ID (#11455)

This PR ports the test suite from the legacy `Decoder` to the new
`StreamDecoder`, to ensure they are fully compatible.

In doing so, I stumbled upon a bug where the `StreamDecoder` would enter
an obscure irrecoverable state when faced with an out-of-order legacy
store ID (i.e. a store ID missing an app ID).
This PR fixes that.

Also removed some duplicated code on the way.

---

This PR is part of an upcoming series of PRs to pay off organic growth
debt in our encoding/decoding stack.

* DNM: requires #11452

Author: teh-cmc

crates/store/re_log_encoding/src/codec/file/mod.rs

@@ -23,7 +23,7 @@ impl MessageKind {
 }
 
 #[allow(dead_code)] // used behind feature flag
-#[derive(Debug, Clone, Copy)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub(crate) struct MessageHeader {
     pub(crate) kind: MessageKind,
     pub(crate) len: u64,